This invention relates to bladed rotor assemblies of the type used in compressors and turbines of turbomachinery. More particularly, the invention relates to circumferential blade root fixings of the type employed in axial-flow compressors and turbines.
British Patent No. 1,187,227 describes a bladed rotor assembly wherein blades each with a dovetail shaped root portion, are loaded into a correspondingly shaped circumferentially extending retaining groove in the rotor hub via a loading slot in the groove. Each blade is shuffled circumferentially around the retaining groove to allow the next blade to be loaded. Usually, in this type of assembly, the final few blades to be loaded have modified platforms in order to obturate the loading slot from the gas flow through the turbomachine thereby preventing a reverse gas flow under the blade platforms adjacent the loading slot. A locking device prevents the blades moving along the groove during operation of the turbomachine.
The loading slot significantly raises the level of stress in the rotor hub and is therefore a limitation of the length of its life. A further cause of increased stress is the need to dimension the loading slot and blade root portions to half the length of the blade platform so that no part of the blade root is aligned with the loading slot after the rotor has been assembled. Ideally the blade root should extend all the way along the platform in order to make the blade as strong as possible.
Modern turbomachinery requires a greater number of blades per hub for a given size of machine for a high efficiency. To achieve this the blade platforms are of a rhombic shape as opposed to the more usual rectangular shape. These so-called `packed rotors` have an inherent problem if the blades are loaded into the hub via a loading slot. Due to clearances between the blade roots and the walls of the retaining operation the blades assume the wrong stagger angle (or angle of attack). In doing so the rhombic shaped platform takes up less room in the circumferential direction than it would at the correct stagger angle thereby allowing extra blades to be loaded into the hub - all taking up the wrong stagger angle.